Ever since the launch of the Telstar satellite, more than fifty years ago, many artificial satellites have been launched into a variety of Earth orbits to be used as communication relay stations. Such communication satellites have achieved, through the years, great improvements in capabilities and cost, providing, in many cases, communication connectivity to regions of the Earth that are poorly serviced by other communication technologies.
The orbit of a communication satellite is usually chosen to maximize the satellite's effectiveness for communications. For example, a communication satellite might be placed in low-earth orbit (LEO) to achieve a short delay for radio communications.
FIG. 1 depicts a polar LEO orbit for a communication satellite. FIG. 1 shows an outline of planet Earth 110, with its day side 120 illuminated by sunlight 130, while the night side 140 is dark. FIG. 1 shows a possible polar LEO orbit 150 for a communication satellite. A polar orbit is understood in the art to be an orbit that passes above or near the polar regions of Earth. The particular polar LEO orbit shown in FIG. 1 can be observed to be, in part, on the night side of Earth. As a consequence, a satellite in such an orbit might be shadowed from the Sun by the Earth over portions of the orbit.
It will be clear to those skilled in the art that some satellite orbits can be exposed to sunlight over their entirety, while others can have substantial portions that lie in the Earth's shadow. This is especially true of LEO orbits. Furthermore, as the Earth goes around the Sun, the angle with which sunlight reaches Earth changes. Also, a satellite's orbit evolves; most notably, the plane of the orbit precesses around the Earth's axis of rotation. Because of these time-dependent changes, an orbit that is entirely in the Sun at some point in time, might be partly in Earth's shadow just a few months later, and vice versa.
Sunlight, of course, has the effect of heating up a communication satellite. Furthermore, the satellite comprises many components that generate waste heat, such as radio amplifiers, batteries, reaction wheels, and others. Such heat, together with heat from sunlight, must be removed in a controlled fashion, so as to keep the temperature of the satellite and its components within a desired range. The heat flow into and out of a satellite from all heat sources is referred to as the “thermal budget” of the satellite. Controlling the thermal budget is referred to as “thermal management”. Most frequently, the goal of thermal management is to stabilize the temperature of the satellite or of critical satellite components.
Air is commonly used for thermal management on the surface of the Earth. For example, automobiles, computers, and many other devices are equipped with fans for cooling. Adjusting the amount of cooling provided by fans can be easily accomplished by, for example, turning fans on and off as needed: if the temperature of a device becomes too high, a fan can be turned on to move air and cool the device. When the temperature goes down, the fan can be turned off.
In space, where there is no air, cooling an object such as a satellite is more difficult. In practice, radiative cooling is the only viable option. Radiative cooling is based on the fact that warm objects emit (radiate) heat (hereinafter “radiant heat”). The warmer the object, the more radiant heat is emitted. Radiant heat is composed of electromagnetic radiation (such as light and radio waves). Indeed, the heating that an object experiences when exposed to sunlight is due to the radiant heat inherent to the sunlight. Radiant heat emissions can be exploited for thermal management of a satellite.
Satellites are usually equipped with thermal radiators. These are satellite components made to be efficient emitters of radiant heat. Because of the physics of radiant heat, thermal radiators are frequently also good absorbers of the radiant heat that comes from hot sources such as sunlight. Satellites are also usually equipped with thermal shields, which are the counterpart of thermal radiators. They are, typically, poor absorbers as well as poor emitters of radiant heat. They can be regarded as thermal insulation for spacecraft.
A satellite might be designed with a heat shield on one side and a thermal radiator on the opposite side. If the satellite is oriented such that the heat-shield side faces the Sun, while the thermal-radiator side faces empty space, radiant heat absorption from the hot sunlight is reduced, while radiant heat emission from the satellite to space is enhanced, and the satellite's temperature can be expected to fall. Conversely, if the satellite is oriented such that the thermal-radiator side faces the Sun, while the heat-shield side faces empty space, radiant heat absorption from the hot sunlight into the satellite is enhanced, while radiant heat emission from the satellite to space is reduced, and the satellite's temperature can be expected to rise. Intermediate orientations will achieve intermediate results, and the temperature of the satellite can, therefore, be controlled simply by adjusting the orientation of the satellite.
As an alternative to adjusting the orientation of the entire satellite, a satellite can be equipped with thermal radiators whose orientation can be adjusted relative to the body of the satellite. For example, the NASA Space Shuttle was equipped with large thermal panels on the inside surfaces of the cargo-bay doors. Temperature adjustments could be accomplished by adjusting the orientations of the thermal panels relative to the body of the Shuttle, and by adjusting the flow of the coolant that carried heat from various parts of the Shuttle to the thermal panels.
Many types of satellites have constraints on how they can be oriented. For example, satellites for Earth observation might be equipped with cameras or other sensors that must be aimed at the Earth's surface. Such a requirement limits the range of possible orientations of such satellites.
For communication satellites, typically, a satellite comprises one or more antennas that must be accurately aimed at regions of the Earth where communication services are to be provided. An antenna is usually characterized by how it transmits radio signals. A radio signal transmitted by an antenna propagates through space with different strengths in different directions. The geometrical shape corresponding to the spatial distribution of the propagating signal is often referred to as the “antenna beam”. The use of the word “beam” reflects the shape of the geometrical shape: for the highly directional antennas commonly used on communication satellites, the geometrical shape looks very much like the beam of a searchlight. If the radio signals were visible, the antenna would look just like a searchlight.
Even antennas used as receiving antennas are characterized by an “antenna beam”. This is because antennas are reciprocal devices, and, when used for receiving radio signals, an antenna exhibits different sensitivity for radio signals arriving from different directions. The geometrical shape corresponding to the spatial distribution of such varying sensitivity is the same as it would be if the antenna were used for transmission. Thus, for a receiving antenna, the “antenna beam” shows the regions of space from which signals can be received efficiently.
A communication satellite might have, for example, a single antenna with a circular antenna beam. FIG. 2 shows an example of such a satellite in a LEO polar orbit. LEO satellite 210 orbits the Earth in LEO polar orbit 150. The satellite is equipped with radio antenna 220 characterized by antenna beam 230. The conical shape that depicts antenna beam 230 represents the region of space where signals transmitted by the antenna are received with good strength. For a receiving antenna, it represents the region of space where signals can originate and be received by the satellite with good efficiency. The region of the Earth where antenna beam 230 intersects the surface of the Earth is commonly known as “coverage area” and is depicted in FIG. 2 as coverage area 230. It is the region of the Earth where satellite 210 can provide communication services through antenna 220.
In FIG. 2, the antenna beam is depicted as having circular symmetry, such that coverage area 230 is a circle. Furthermore, the antenna is aimed “straight down” meaning that the axis of circular symmetry of the antenna beam meets the surface of the Earth at right angles, and the point where it meets the surface of the Earth is the subsatellite point, shown in FIG. 2 as subsatellite point 240. An axis that goes from a satellite to the Earth and meets the surface of the Earth at right angles is commonly referred to as the “yaw” axis and is depicted in FIG. 2 as yaw axis 250. The circular coverage area 230 is centered around the subsatellite point.
Because of the circular symmetry, LEO satellite can freely rotate around the yaw axis without affecting the shape of the coverage area. Such a maneuver is known as “yaw steering” and can be advantageously used for thermal management of the satellite. The satellite can be equipped with a combination of thermal radiators and heat shields on different sides of the satellite, and can be rotated, as needed, to expose one side or another to sunlight, without affecting the shape and size of coverage area 230.
In practice, yaw steering is not an option for many communication satellites that are equipped with multiple antennas. With such satellites, radio communications occur in accordance with a geometric pattern of multiple antenna beams usually referred to as a “beam pattern” of the satellite. The beam pattern must be accurately aimed at the surface of the Earth to generate a desired pattern of coverage areas. Patterns of coverage areas are carefully designed to achieve a desired performance, and any disruption of the patterns can be very harmful. This is especially true when the satellite is part of a satellite constellation where the communication services provided by one satellite must be coordinated with the communication services provided by other satellites.
In many communication satellites, the antennas are rigidly affixed to the body of the satellite. The relative positions and relative orientations of the antennas are designed to achieve a desired beam pattern; i.e., a beam pattern which, when properly aimed at the surface of the Earth, results in a desired pattern of coverage areas. Rotation of the satellite around the yaw axis, or any other axis, is accompanied by a rotation of the beam pattern. Any substantial rotation typically results in an unacceptable distortion of the pattern of coverage areas. Therefore, thermal management through yaw steering is not an option for such satellites.
One possible solution is to use steerable antennas. Such antennas generate beams whose orientation can be adjusted. Both mechanical and electronic means for beam steering are possible. The satellite can then be rotated while the antennas are steered so as to maintain the desired beam pattern.
Another possible solution is to equip the satellite with more adaptable thermal radiators. For example, the thermal radiators might be mounted on the body of the satellite with gimbals and motors that enable them to be moved, relative to the body of the satellite. Their angle of exposure to sunlight can then be changed as needed to achieve the desired thermal management. Additionally, thermal radiators can be equipped with thermal switches that disable radiators when necessary, or with a system for circulating coolant to the panels wherein the coolant circulation pattern is made adjustable by means of valves and pumps as needed to achieve the desired thermal management.
These solutions and others available in the prior art have significant disadvantages of added cost, added satellite weight and inhibited performance. In the future, communication satellite systems with small, light, compact, low-cost satellite designs will provide a range of new services. To make such satellites a reality, a different method for thermal management is needed.